1. Tokyo Research Center, Kao Corporation, 2-1-3 Bunka, Sumida-ku, Tokyo 131, Japan, Recording and Imaging Science Laboratories, Kao Corporation, 2606, Akabane, Ichikai-machi, Haga-gun, Tochigi 321-34, Japan, and Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139